Convert ranger uses of real_inf to dconst[n]inf.

gcc/ChangeLog:

	* range-op-float.cc (build_le): Convert to dconst*inf.
	(build_ge): Same.
	* value-range.cc (frange::set_signbit): Same.
	(frange::normalize_kind): Same.
	(range_tests_floats): Same.
	* value-range.h (vrp_val_max): Same.
	(vrp_val_min): Same.
	(frange::set_varying): Same.